A piston in a liquid-pressure rotating device in which pistons are arranged at a cylinder block rotatable together with a rotating shaft, includes: a cylindrical peripheral wall main body portion including a hollow portion, an opening portion formed at one end of the body portion and communicating with the hollow portion; a lid portion welded to an inner peripheral surface of the opening portion sealing the hollow portion and including an outer surface formed as a pressure receiving surface which receives liquid pressure; and a stress reducing portion provided on an inner peripheral surface of the body portion, the inner peripheral surface in contact with or close to an end portion of a welded portion formed between the opening portion and the lid portion, the end portion located at the hollow portion side, the stress reducing portion configured to reduce stress acting on the end portion of the welded portion.